Pivot mechanism with electronic dither circuit

ABSTRACT

Pivot mechanism with electronic dither circuit having a housing, a member disposed in the housing and means pivotally mounting the member in the housing for pivotal movement about a pivot axis. Means is connected to the member for moving the member about the pivot axis and means is utilized for applying oscillatory transitory movement to the member to minimize friction in the means pivotally mounting the member in the housing.

Unitefl States Patent 1191 Morris Mar. 19, 1974 1 PIVOT MECHANISM WITH ELECTRONIC 3.062.057 I 11/1962 Glaser et al 73/432 A DITHER CIRCUIT 2.821610 3/1958 Hiser 324/155 I I I I 3.382722 5/1968 Bridge et a1. 73/432 A [75] Invent r: H r M mda. Calif. 2.427.180 9/1947 Ballard 324/155 ux [73] Assigneez systrnmn n Corporation, 2.511.485 6/1950 Strobel .1 324/125 Concord, Calif. j 1 I Primary Examiner-James J1 Gill [22] Fll l M y 1972 Attorney, Age/11,01 Firm-Flehr, Hohbach. Test, Al- 21 Appl. No.: 251,392 br'tton & Herbert Related US. Application Data I 57] ABSTRACT [63] Continuation of Sert No.I47,27l, June 18, 1970,

abandoned Pivot mechanism with electronic dither circuit having a housing, a member disposed in the housing and 52 us. c1 73/517 B means piverally mounting the member in the housing 51 Int. Cl. GOlp 15/08 for pivotal movement about a pivot aXis- Means is 58 Field of Search 324/155, 125; 73/517 R, eenneeted to the member for moving the member 73/517 B 496 432 A about the pivot axis and means is utilized for applying oscillatory transitory movement to the member to [56] R fe ci d minimize friction in the means pivotally mounting the UNITED STATES PATENTS member housmg' 2.914712 11/1959 Chamberlain 324 155 11 Claims, 2 Drawing Figures I "I 11 32 {*3/ N 11 I n s lst ivii 1 l I AIQFEFDBACK AMPLIFIER NETWORK I 5; I 27 eeocx I 23 37 3 I FIIECTIFIER 33 I :r I II 1 -vwvvv- OSCILLATOR DIFE OPER. OUTPUT I 1 AMP BALANCE AMF. AMP I 4 NETWORK OSCILLATOR 7 2 COUPLING I 35 5 J F 1 SYSTEM 22 RECTIFIER 37 l TORQUE 1 i} 1 l} c011.

PICKOFF 1 I LOAD I L E.

c011. l NETWORK 1 ACCEL g I NZ 51Gb: 5: I 17 l l 43 IOUT" l j 5E LOAD I POSITION J I JRESISTOR I m n -7 DETECTOR OSCILLATOR I I In La," I I J L. .l I .52 -153 TIME VOLTAGE 'DELAY DET DITHER BLOCK PIVOT MECHANISM WITH ELECTRONIC DITHER CIRCUIT This application is a continuation of application Ser. No. 47,271 filed June 18, 1970, now abandoned.

BACKGROUND OF THE INVENTION In pivot and jewel movements of all types, there is a turn-on stiction which must be overcome. Such turnon stiction is the static friction which is encountered in commencing movement of one of the parts relative to the other in a pivot mechanism. Turn-on stiction is found in many types of instruments. For example, it can be found in meters using DArsonval movements. It also can be found in accelerometers which pivotally mount the pendulous mass. In meters and accelerometers, attempts have been made to minimize this tumon stiction by tapping mechanically or by hand the meter or the accelerometer. In general, such a procedure has been found'to be relatively unsatisfactory. There is, therefore, a need for a new and improved pivot mechanism with means for overcoming turn-n stiction.

SUMMARY OF THE INVENTION AND OBJECTS The pivot mechanism with electronic dither circuit consists of a housing and with a member disposed within the housing. Means is provided for pivotally mounting the member in the housing for movement about a pivot axis. Means is connected to the member for moving the member about the pivot axis. Means is also provided for applying oscillatory transitory move ment to the member to minimize stiction in the means for pivotally mounting said member.

In general, it is an object of the present invention to provide a pivot mechanism with an electronic dither circuit which minimizes stiction in the pivot mechanism. 1

Another object of the invention is to provide a pivot mechanism of the above character which can be utilized in many different types of instruments.

Another object of the invention is to providea pivot mechanism of the above character which is relatively simple.

Another object of the invention is to provide a pivot mechanism with an electronic dither circuit of the above character in which dithering can be applied at spaced time intervals.

Additional objects and features of the invention will appear from the following description in which the pre ferred embodiment is set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT In copending application Ser. No. 47,270 filed June 18, 1970 now U.S. Pat. No. 3,707,091 there is disclosed dual pick-off electronic circuitry for use with linear servo accelerometer. This same electronic cirfor pivotally mounting the member 2 within the housing and consists of a pair of pivot and jewel assemblies 3 and 4 disposed on opposite sides of the pendulous member 2. A planar conducting element in the form of a paddle 11 is carried by the member 2. First and second pick-off coils 16 and 17 are disposed on opposite sides of the paddle 11 and are provided for sensing the position of the paddle 11. First and second oscillators 19 and 20 are provided and are connected to the first and second pick-off coils so that the first pick-off coil forms a part of the tank circuit of the first oscillator and that the second pick-off forms a part of the tank circuit of the second oscillator. Means is provided in the form of a resistor 22 which interconnects'the first and second oscillators so that the first and second oscillators operate in synchronism at substantially identical frequencies. The outputs of the oscillators are rectified in first and second rectifiers 23 and 24. The outputs of the first and second rectifiers are supplied to a differential amplifier 26 having two inputs and two outputs. A.C. feedback means 27 and 28 are provided for connecting the outputs of the differential amplifier to the oscillators. The ac. feedback means include a dc. blocking capacitor 29, a coupling resistor 31 and a temperature compensating thermistor 32. Means is provided for converting the double-ended output from the differential amplifier into a single-ended output and consists of an operational amplifier connected. to the output of the differential amplifier. A torque coil 6 is provided within the housing and is mounted on the pendulous member 2. The output of the operational amplifier is supplied to the torque coil which applies forces to the pendulous member 2 to restrain movement of the paddle 1 1. The circuitry also includes a balance network 34 which balances the output from the differential amplifier 26. Additional amplification is provided for the operational amplifier in the form of an output amplifier 36. A gain set resistive circuit 37 is connected between the output of the output amplifier 36 and one of the inputs to the operational amplifier 33. The torque coil 6 together with a load resistor 43 is represented as a load network 411.

The electronic dither circuit for the pivot mechanism of the accelerometer is shown in the block 51 and, as hereinafter described, is utilized to minimize the initial effects of friction during turn-on of the accelerometer. The dither circuit consists of a second'order closedloop servo system with a very small phase margin. To dither circuit 51 includes a time delay circuit 52 which supplies its output to a voltage detector or comparator circuit 53 and the output of the voltage comparator is supplied to a resonator or dither circuit 54. The output of the integrator 56 is connected to one of the oscillators 19 and 20 to the emitter of the transistor and between the serially connected capacitor of the tank circuit of the oscillator. The resonator 54 consists of an integrator 56 of a conventional type which supplies its output to a lag circuit 57.

As can be seen from the circuit diagram of the dither circuit shown in FIG. 2, the time delay circuit 52 consists of a transistor Q3 which is connected as a grounded base amplifier that supplies a constant collector current to a timing capacitor C104. The base of the transistor is connected to ground through the resistor R111. The emitter of the transistor O3 is connected to a plus 22 volt dc line by a resistor R102. The 22 volt dc line is connected to the base of the transistor Q3 through a pair of diodes CR101 and CR102. The collector of the transistor O3 is connected through a resistor R110 to one terminal of the field effect transistor Q4. One terminal of the transistor O4 is connected to a terminal of the uni-junction transistor Q5. Another terminal of the transistor O5 is connected to ground as shown. The remaining terminals of transistor Q4 and OS are connected through a resistor R103 to the plus 22 volt dc supply. The transistor Q4 and Q5 serve as the voltage detector 53 which supplies its output to one terminal of the resonator 54.

The integrator 56 of the resonator 54 consists of a resistor 112 connected to the negative input terminal of a conventional integrated circuit amplifier 59. The output of the amplifier 59 is connected through an integrating capacitor C107 to the negative input terminal of the amplifier 59. The output of the amplifier 59 is suppliedto the lag circuit 57. The lag circuit 57 consists of a resistor 113 which is connected to the gate terminal of field effect transistor Q6. Capacitor C105 is connected between the gate terminal of Q6 and ground. One terminal of the transistor O6 is connected to ground as shown. The other terminal is connected to a 22 volt dc line through a resistor R105. It is also connected through a resistor R104 to'the negative input terminal of the amplifier 59.

When the power is turned on to the dither circuit, a small fixed voltage is generated by current passing through the diodes CR101 and CR102, in series through resistor R111 to ground. This small fixed voltage is supplied to the base of the transistor Q3 and leaves a requirement with a single diode voltage or onehalf of the small fixed voltage be produced across the resistor R102. The transistor Q3 operates as an emitterfollower and therefore reproduces a voltage supplied to its base circuit. One-half of the small fixed voltage appears in going between the base and the emitter circuit of transistor 03 and leaves the remaining one half of the small fixed voltage across the resistor R102. As long as it is possible for current to flow in the collector circuit I of the transistor Q3, the transistor Q3 will produce a constant current flow across the resistor R102. This current is used for charging the timing capacitor C104.

At power turn-on, the timing capacitor C104 is without a charge and therefore has a voltage of zero. As current passing through the transistor Q3 builds up a charge in the form of voltage on the capacitor C104, it builds up to a point to discharge through the resistor R110 to cause operation of the field effect transistor Q4. This causes operation of the unijunction transistor Q5 which then produces a step transient through the resistor R1 12 into the resonator or dither of the dither circuit. The integrated circuit amplifier 56 acts as an integrator with an integrating capacitor C107 across the amplifier. The output of the integrator is supplied through a resistor R113 to a timing capacitor C105.

The resistor R113 and C constitute the second time constant on the lag circuit. The voltage across the capacitor C105 is reproduced by a source follower in the form of the field effect transistor Q6 where from source to ground, the voltage produced .on the output of the transistor Q6 exactly reproduces the voltage on C105 and is again applied to the input of the integrator through resistor R104. Thus, the resistor R104 serves as the input resistor for the integrator and capacitor C107 serves as the integrating capacitor and resistor R113 serves as the resistor part of the RC lag circuit 57 with the capacitor C105 being the storage element in the lag circuit. A transistor Q6 serves as an impedance matching device by applying the voltage on capacitor C105 back to the integrator, without reflecting a significant loading on the lag circuit 57.

Thus, it can be seen there is provided a closed loop oscillator which almost reaches the criteria for oscillation but is guaranteed not to by the choice of constants and the number of time constants in the circuits so that for a given transient applied through resistor R112 by the timing circuit there will be a given response in terms of a decaying sinusoidal output. This sinusoidal output is applied through a large capacitor C103 through a resistor R which converts the output into a current which flows into the emitter of the transistor of the oscillator to which the dither circuit is con nected.

The total phase shift through the closed loop oscillator is always slightly less than so that the system will never oscillate freely but instead will produce damped oscillations. The frequency of the damped oscillation for a phase margin of less than one per cent (1%) is very close to that frequency for which the gain around the loop is equal to one.

Operation of the dither circuit may now be briefly described as follows. The first transient is produced by the dither circuit when the power is turned on which causes the sudden application of voltage to the resonator circuit 54. The resonator then produces an oscillatory transitory signal in the form of a decaying sinusoidal output which is applied to the oscillator 19 to modulate the oscillator by current injection. This sinusoidal voltage causes the current to flow which modulates the oscillator at a predetermined frequency as, for example, 5 cycles per second and causes the accelerometer moving system through a servo action as described in copending application Ser. No. 47,270 filed June 18, 1970 to move back and forth through null and to null out the disturbance which has been inserted into the oscillator 19.

More specifically, the operation of the dither circuit in causing the mechanical system to move is as follows. The oscillator 19 is running as a grounded base. The current flowing in the emitter of the transistor of oscillator 19 represents the sum of the currents at the junction of the emitter with the other parts of the oscillator 19. Thus, the emitter current is the sum of three separate currents at this junction which are as follows:

a. a constant direct current from the battery through the resistor and through the junction;

b. a variable alternating current through the AC. feedback and through the junction;

c. a variable alternating current from the dither circuit and through the junction.

The latter two variable currents modulate the transconductance of the transistor of oscillator 19 to vary the loop gain of the oscillator and thus electrically modulate the oscillator 19. This electrical modulation of the oscillator 19 is cancelled out because the oscillator 19 supplies energy to the torque coil 6 and thereby causes mechanical movement of the mechanical system in which the paddle llll is moved relative to the pick-off coils in and 117 to modulate the oscillator 19 to cancel the electrical modulation.

in this movement of the moving system of the accelerometer, the bearing pivot interface is oscillated at the frequency at which the oscillator is modulated by the sine wave generated by the output from the resonator 54. This sine wave starts initially at a large valve and decays away gradually over a predetermined period of time as, for example, 20 seconds.

The modulation of the oscillator which occurs is similar to the generation of a transient which gradually decays. By supplying a step of transient excitation to the resonator 5d, the output from the resonator 54 will ring in the same manner as a high Q tuning fork except it does it electronically with the output being the voltage waveform on the output of the integrator. This is very similar to the amplitude of the tuning fork which gradually decays away to zero.

With the dither circuit shown, a transient is applied to the integrator 56 at the time power is turned on which causes the dither circuit to produce an exponentially decaying sinusoidal waveform to the oscillator 19. After the time delay 52. runs out, it operates the voltage comparator or detector 535 to start the resonator 54 ringingagain. This ensures that the effects of stiction are minimized. The time delay circuit 52 applies another step transient to the resonator 54 and causes another exponentially decaying sinusoidal waveform to be applied to the oscillator 19 after a predetermined period of time which would preferably be a period of time which is sufficiently long to ensure that all of the electronic circuitry had warmed up and had reached its steady state operating condition.

Thus, it can be seen that the dither circuit operates twice, once when the power is first applied to the devices on which it is being used to cause the moving system to be moved back and fourth to overcome the effects of friction in the pivots and jewels provided for mounting the moving ststem and at a second time which is sufficiently delayed from turn-on so that the electronics have reached a stable or steady-state condition to ensure that the effects of friction have been minimized. On both applications of the sinusoidal waveform, the sinusoidal waveform decays relatively rapidly so that the mechanism on which the dither circuit is being utilized can quickly return to its normal opera tional mode.

By way of an example, a dither circuit constructed in the following manner had a ringing frequency of nominally 5 Hz in response to a voltage step input.

The capacitor C W3 is relatively large. However, it is desired to minimize direct current leakage through this coupling capacitor. This is accomplished by biasing the coupling capacitor in such a manner as to keep the dc. potential across the capacitor to a less than a predetermined value as, for example, 200 millivolts. Such biasing is accomplished by referencing the signal ground of the dither circuit to the same potential as the emitter of the position detector oscillator to which it is connected. Tracking of the capacitor bias with temperature is assured by proper reference selection and by operating the field effect transistor in the dither loop at a zero temperature coefficient point. ln a linear servo accelerometer, it has been found that a range of excursion from 1.7 to 2.8 milli-radians from null was found to be satisfactory to eliminate the phenomenon known as turn-on stiction in the moving system pivot and jewel bearings. By the utilization of such a dither circuit, it was found that the moving system eventually came to rest at the desired null position without significant error due to tum-on stiction.

Although the electronic dither circuit has been described in conjunction with a pivot mechanism of a linear servo accelerometer of the type having a dual pickoff, it is readily apparent that the same dither circuit can be utilized in conjunction with a single pick-off. In a single pick-off mechanism, the dither circuit could be utilized periodically to remove any effects of drift. The electronic dither circuit can be utilized with any type of mechanism inwhich a pivotal mounting is provided for a portion of the mechanism. Thus, for example, it can be utilized in conjunction with open loop type instruments as, for example, a meter. The dither circuit could be utilized for accomplishing the same thing as is done by tapping the instrument to eliminate the effects of friction. The electronic dither circuit would have the advantage in that it would be far more reproducible and certainly would be more suitable for a precision measuring instrument where it is desired to minimize as much as possible any of the effects of friction before taking a reading from the instrument. At each reading of the instrument, the electronic dither circuit could be operated to minimize the effects of friction. It could be seen that theelectronic dither circuit removes the effects'of friction immediately after turn-on of the instrument and the very tight servo loop in the mechanism eliminates the necessity for any future use as long as the power remains on on the instrument.

It should be appreciated that the electronic dither circuit could be utilized in conjunction with a timing circuit wherein the dithering could be applied periodically.

It is apparent from the foregoing that there has been provided a pivot mechanism with an electronic dither circuit which makes it possible to greatly improve the accuracy of the instrument in which the pivot mechanism is utilized. It serves to eliminate or at least minimize any error in the sensing mechanism which would occur because of turn-on stiction in the moving system bearings.

ll claim:

1. ln a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis and electronic means for applying a modulated electrical signal to said coil to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces a decaying sinusoidal output.

2. A mechanism as in claim 1 wherein said electronic means includes means for applying said oscillatory transitory movement to the member at the time that the mechanism is placed into use and at a second period of time.

3. A mechanism as in claim 1 wherein said electronic means gradually reduces the transitory movement to zero.

4. A mechanism as in claim 1 wherein the transitory movement is an exponentially decaying function.

5. In a servo accelerometer, a housing, a pendulous member, means pivotally mounting said pendulous member in said housing, a torque coil carried by said pendulous member, means for establishing a magnetic field in the vicinity of the torque coil, a conducting planar element carried by the pendulous member, pick-off means mounting within the housing and disposed in the vicinity of the conducting planar element, an oscillator connected to the pick-off means in such a manner that the pick-off means forms a part of the tank circuit of the oscillator, means for rectifying the output of the oscillator, means for supplying the rectified output to the torque coil, and electronic means for modulating the oscillator to cause the conducting planar element to move back and forth through a null position.

6. An accelerometer as in claim 5 wherein said electronic means for causing said conducting planar element to move back and forth through the null position includes means for supplying a decaying oscillation to the oscillator to modulate the same.

7, An accelerometer as in claim 6 wherein said electronic means for applying a decaying oscillation to said oscillator includes means for supplying a decaying oscillation to said oscillator at two spaced intervals of time, said last named means including time delay means.

8. An accelerometer as in claim 7 wherein one of said intervals of time is at the time that the accelerometer is turned on and the otherof the intervals of time is at a predetermined time after the accelerometer has been turned on. v

9. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis and means for moving the member about the pivot axis, said last named means including electronic means for causing an oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces damped oscillations, said resonator including an integrator, a lag circuit coupled to the output of the integrator and a feedback loop coupling the output of the lag circuit to the input of the integrator.

10. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis and electronic means for applying a modulated electrical signal to said coil to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces damped oscillations, said transitory movement being in the form of an exponentially decaying function.

11.'In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis, means for sensing the position of said member in its pivotal movement and electronic means for applying a modulated electrical signal to said coil'to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a lightly damped closed-loop servo system of at least the second order type coupled to said means for sensing the position of said member in its pivotal movement. 

1. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis and electronic means for applying a modulated electrical signal to said coil to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces a decaying sinusoidal output.
 2. A mechanism as in claim 1 wherein said electronic means includes means for applying said oscillatory transitory movement to the member at the time that the mechanism is placed into use and at a second period of time.
 3. A mechanism as in claim 1 wherein said electronic means gradually reduces the transitory movement to zero.
 4. A mechanism as in claim 1 wherein the transitory movement is an exponentially decaying function.
 5. In a servo accelerometer, a housing, a pendulous member, means pivotally mounting said pendulous member in said housing, a torque coil carried by said pendulous member, means for establishing a magnetic field in the vicinity of the torque coil, a conducting planar element carried by the pendulous member, pick-off means mounting within the housing and disposed in the vicinity of the conducting planar element, an oscillator connected to the pick-off means in such a manner that the pick-off means forms a part of the tank circuit of the oscillator, means for rectifying the output of the oscillator, means for supplying the rectified output to the torque coil, and electronic means for modulating the oscillator to cause the conducting planar element to move back and forth through a null position.
 6. An accelerometer as in claim 5 wherein said electronic means for causing said conducting planar element to move back and forth through the null position includes means for supplying a decaying oscillation to the oscillator to modulate the same.
 7. An accelerometer as in claim 6 wherein said electronic means for applying a decaying oscillation to said oscillator includes means for supplying a decaying oscillation to said oscillator at two spaced intervals of time, said last named means including time delay means.
 8. An accelerometer as in claim 7 wherein one of said intervals of time is at the time that the accelerometer is turned on and the other of the intervals of time is at a predetermined time after the accelerometer has been turned on.
 9. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis and means for moving the member about the pivot axis, said last named means including electronic means for causing an oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces damped oscillations, said resonator including an integrator, a lag circuit coupled to the output of the integrator and a feedback loop coupling the output of the lag circuit to the input of the integrator.
 10. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis and electronic means for applying a modulated electrical signal to said coil to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a resonator which produces damped oscillations, said transitory movement being in the form of an exponentially decaying function.
 11. In a pivot mechanism with an electronic dither circuit, a housing, a member disposed in the housing, means pivotally mounting the member in the housing for movement about a pivot axis, electromagnetic means mounted in the housing including a coil for moving the member about a pivot axis, means for sensing the position of said member in its pivotal movement and electronic means for applying a modulated electrical signal to said coil to cause oscillatory transitory movement of the member to minimize the effects of friction in the means for pivotally mounting the member, said electronic means including a lightly damped closed-loop servo system of at least the second order type coupled to said means for sensing the position of said member in its pivotal movement. 